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Show Journal of Neuro- Ophthalmology 14( 3): 18$, 1994 '• 1994 Raven Press, Ltd., New York Letters to the Editor To the Editor: Slavin's Prism Dissociation test is extremely useful in the evaluation of suspected functional unilateral visual loss ( 1). I have also been using a variation on that theme. With the patient viewing binocularly behind the phoropter, the Risley prism is positioned in front of the eye with the alleged visual loss, so that vertical prism can be introduced at will by the examiner. Several lines of the Snellen chart are displayed simultaneously. It is helpful here to have a projected chart and a slide that has several 20/ 20 lines, but the display should in any case only include letters at least two or three lines smaller than the claimed acuity of the visually impaired eye, to give the patient the benefit of the doubt. The patient is then asked to begin reading the display slowly from the top down. When the second or third line is reached, some vertical prism is introduced. If the visual loss in functional, there will suddenly be a great deal of hesitation as the patient's vertical fusional capacity is approached and exceeded ( usually about 3- 4D), since the diplopia will result in dissimilar lines being superimposed on each other. As additional prism is then introduced, the patient is asked innocently what the problem is. In most cases the patient will admit that there are now two charts. After a response implying concern ( a simple surprised " Oh?!" will do), the examiner then asks the patient to continue reading the lower chart ( if base- up prism has been introduced). Usually this will allow quantitation of the true acuity of the " bad" eye. I have found this quite useful. In particular, the sudden hesitation and difficulty reading on the part of functional patients is quite unmistakable. In contrast, the patient with genuine unilateral visual loss will continue to read the chart fluently, unaware that prism has been introduced. Leah Levi, M. B. B. S. Departments of Ophthalmology and Neurosciences University of California, San Diego Reference 1. Slavin ML. The prism dissociation test in detecting unilateral functional visual loss. / Clin Neuro- ophthalmol 1990; 10: 127- 30. To the Editor In the last two patients I have seen with superior oblique myokymia, the phenomenon was audible, with a stethoscope over the affected eye. It sounded just like the firing patern of neurons when it is played over a loudspeaker during neurophysio-logic experiments, a somewhat irregular rapid machine- gun sound, quite distinct from the faint white noise that is normally audible over the eye and also from the occasional noise made by small saccades. Having the patient open the nonauscul-tated eye diminishes any noise from the lid muscles. In the first patient, I came upon this by accident, having decided to auscultate the eye for some reason. I asked my colleagues at the New York Neuro- Ophthalmology Forum whether anyone had heard this phenomenon, but none had, and I do not recall having seen it mentioned in the literature on superior oblique myokymia. The second patient was a 37- year- old woman who complained of intermittent oblique diplopia for the last IV2 years. This occurred from once every few days to many times a day and was typically aborted by changing her gaze in some fashion. She also noticed occasional vertical shimmering of her right eye vision. She felt that the diplopia occurred more if she was tired or under stress and that reducing her coffee intake diminished its frequency. On examination, intermittent right superior oblique overaction was provoked by down and left gaze, with a simmering of the Mad-dox rod image from the right eye. At other times there was no vertical deviation or oscillopsia. At the slit- lamp, a fine vertical- torsional shimmering of the right eye could be seen sometimes on down and left gaze, and at the same time she saw fixation light double, with simmering of the upper image. The rest of the examination was unremarkable. Auscultation of the left eye was normal, with a slight increase in white noise on down and right gaze. Auscultation of the right eye was normal straight ahead, but the firing pattern described above was audible on down and left gaze and was quite distinct from the slight increase in noise audible over the other eye with the corresponding maneuver. I tried to record the phenomenon with a home tape recorder, but it did not come through on the tape. I do not know what the source of the sound is. It would seem unlikely that neuronal firing would be heard, but perhaps the twitching of the muscle fibers or the movement of the tendon or globe can be heard. This phenomenon might occasionally be a useful adjunct in the diagnosis of superior oblique myokymia, although generally once one thinks of it, it is not difficult to make the diagnosis. Christian Wertenbaker, M. D. 379 City Island Avenue Bronx, New York |
